The fraction of total cell-associated Fe59, given as a pulse of transferrin, that becomes ferritin bound is proportional to the actual ferritin level and is independent of the amount of iron supplied or taken up. This has allowed us to establish a nomogram that correlates different levels of intracellular ferritin with corresponding percentages of incoming iron delivered to ferritin. A partition function has been established for the distribution of iron to ferritin. In addition, the T1/2 for the protein moiety of ferritin is of the order of 4-5 hours--a value which questions the conventional view of ferritin as an inert storage protein.